The present invention relates to an identification method for the cuff of a non-invasive automatic sphygmomanometer and to a flow restriction means required in the cuff size identification method for moderating the rate of pressure increase or decrease or both. This type of flow restriction means is particularly useful in connection with small-sized cuffs intended for newborn babies. However, when using larger, adult cuffs it is preferred that the flow restriction means be omitted completely or, if such a means is indeed needed, its gas or liquid through-flow must be sufficiently different from the flow restricting capacity of a flow restriction means used in connection with a smaller cuff. As a result of the generation of pressure occurring at the end of pumping, the size of a presently used cuff is identified.
In some equipment, the selection of cuffs intended for adults and newborn babies is effected by means of a selector switch provided on the device. Thus, the selector switch is used manually for selecting a mode of operation for the device depending on whether the question is about an adult or a newborn baby. There is however a hazardous possibility of forgetting to change the position of a selector switch when substituting e.g. an infant cuff for an adult one. Such a lapse might cause serious damage to newborn babies.
EP Patent application No. 122123 discloses a method developed for automatic identification of the size of a cuff. A certain pressure is pumped in the cuff followed by opening a valve for the reduction of cuff pressure. Opening of the valve generates a sonic pulse which travels along a tube to the cuff and further to a sensor which calculates the time between opening of the valve and arrival of the sonic pulse in the sensor. The arrival time of a sonic pulse in the sensor varies according to the size of a cuff. This delay time of a sonic pulse can be further modified by reducing the length of a tube leading to an infant cuff as compared to the length of a tube leading to an adult cuff. The delay time of a sonic pulse can be further cut down by fitting the tube leading from valve to cuff with a branch tube directly to the tube leading from cuff to sensor. On the basis of the delay time of a sonic pulse, a microprocessor detects whether the question is about an adult or an infant cuff.
In the cited EP Patent application, one problem in the solution based on a sonic pulse is that it is not possible to use tubes of arbitrary length leading from pressure transmission system to cuff since, in a program according to this system, a delay time is measured for the pulse and certain acceptable limits are set for that time. However, certain practical cases require tubes that should be either substantially shorter or longer in length than the tubes used in connection with the cited invention. The cuffs intended for adults and children may not be safely fitted with tubes of equal length as this may complicate the identification of the size of a cuff.
Another problem is also that, when a cuff is to be replaced with another one of different size, the entire long tube must also be replaced. The use of a short adapter cannot be managed.
A pipe linking the tube extending from pressure transmission system to cuff to the tube extending from cuff to sensor, said pipe cutting down the time used by a sonic pulse from valve to sensor, carries the disturbances caused by pumping and opening of the valve directly to the sensor. For this reason, the measuring accuracy suffers particularly in the case that cuff pressure control in the system is a continuous action.
Also known is a method, wherein a certain pressure is pumped in a cuff and, after reaching this pressure, the time taken up by pressure reduction is measured. This idea is based on the fact that the outflow of air is dependent on the size of a cuff. The longer the time taken up by outflow, the larger the cuff being used. A problem in this solution is the blocking of a cuff or a cuff tube, which in the worst case may lead to incorrect identification of the size of a cuff. Blocking of the tube can considerably lengthen the time taken up by the deflation of a cuff.
Attempts have been made to solve this last-mentioned problem by fitting the tube leading from pump and valve to cuff with a tubing which, in turn, is connected at its other end to the tube extending from cuff to pressure sensor and, thus, by-passing the cuff. This connecting tube, in turn, creates problems in connection with measurements by carrying the disturbances caused by pumping and opening of the valve to the pressure sensor. Measuring accuracy suffers especially in the case that cuff pressure control in the system is a continuous action.
The automatic identification of a cuff size can also be effected by measuring the time required for pumping a certain pressure in the cuff. Thus, the pumping time in connection with a larger cuff is longer than in the case of a smaller cuff. Problems in this type of solution include disturbances in the mains and variation of the pump characteristics which may lead to serious errors in the identification of a cuff.